To really hear the advantage of Schade feedback; omit C3 and D1 on the driver's cathode.

Hmmm... Hadn't thought of that. I'll give that a whirl.

D1 is actually not populated by default. That's why it says "DNP" = Do Not Populate on the schematic. I included it as an option. I actually get quite good performance with a green LED in that spot. Then you'll need R9 = 0 Ω and R10 = DNP.

6LU8's makes a killer PP amp..... I did a PCB concept for Triode, UL or Pentode mode, I did Pentode mode for the first one... Great with 6.6K Iron!. I used a 6CG3 diode for slow B+ start in the power supply filter chain.

I'm nearly done with the evaluation of my prototype. So far the board looks solid. I expect to submit it to the board fab on Monday or Tuesday. The turnaround time is two weeks (including shipping to my door). Once I have the boards in hand, expect it to take three days for shipping within the US and 1~3 weeks for international shipping.

To really hear the advantage of Schade feedback; omit C3 and D1 on the driver's cathode.

Tried that. Wouldn't recommend it.

So... I played around with the feedback resistor. I performed a sanity check with values of R3 both above and below 150 kOhm. R3 = 150 kOhm continues to provide the best performance.

I did find that the THD at higher frequencies was quite a bit higher than at midband, so I decided to investigate this further. It turned out that the transient response at the anode of the driver stage was less than stellar as the feedback from the output stage added significant overshoot. It made me wonder if the output stage was actually operating at the edge of stability.

To tame the transient response, I added a small (22~33 pF) cap in parallel with R3. That worked like a charm. This cap does limit the bandwidth of the amp to about 30 kHz (-3 dB), but its transient response is nice and symmetric without tendency to overshoot.

I need to run the circuit through my measurement suite, but I think this is it folks! Ready for prime time!

Wow. What a difference a sliver of mica makes. Adding a 'speedup' cap to the feedback really tightened up the transient response. Metallic instruments are now rendered with much more detail. Smooth...

The amp sounds good and measures well. The board will be submitted for fabrication by Monday morning. It should arrive on my doorstep by the 29th or 30th of September.

I will probably order a fairly small batch of boards as there doesn't seem to be as much excitement about it as I had anticipated. If you want to get your paws on one of these boards, I suggest getting your order in early.
The board will come with a full set of schematics, bill-of-materials, circuit description, getting started guide, and include the latest tweaks and optimizations.